The invention relates to a method for producing an aluminum alloy heat exchanger.
An aluminum alloy has been normally used for an automotive heat exchanger (e.g., evaporator or condenser) due to its reduced weight and excellent thermal conductivity. Such a heat exchanger has been normally produced by applying a fluoride flux to the surface of an aluminum alloy extruded tube, assembling a member (e.g., fin material) with the aluminum alloy extruded tube to form a given structure, and brazing the aluminum alloy extruded tube and the assembled member in a heating furnace that contains an inert gas atmosphere, for example.
A multiport tube having a plurality of hollow areas (refrigerant passages) that are defined by a plurality of partitions is normally used as an extruded tube used to produce an automotive heat exchanger. In recent years, a reduction in weight of a heat exchanger has been desired in order to reduce the fuel consumption of automobiles from the viewpoint of reducing environmental impact, and a tube used to produce a heat exchanger has been reduced in thickness. Therefore, the cross-sectional area of the tube has been reduced, and a several hundred to several thousand extrusion ratio (cross-sectional area of container/cross-sectional area of extruded product) has been employed. Therefore, a pure aluminum material that exhibits excellent extrudability has been used taking account of the extrusion ratio.
It is expected that the weight of a heat exchanger and the thickness of a tube will be more and more reduced. Therefore, it is necessary to increase the strength of the tube material. It is effective to add Si, Cu, Mn, Mg, or the like in order to increase the strength of the tube material. When the brazing target material contains Mg, a fluoride flux that is melted during heating reacts with Mg in the material to produce compounds such as MgF2 and KMgF3. This reduces the activity of the flux, so that brazability significantly deteriorates. The addition of Cu significantly decreases extrudability, so that the die breaks, or the productivity decreases. Therefore, Si and Mn must be necessarily added in order to increase the strength of the tube material.
When adding Mn and Si to an alloy at a high concentration, Mn and Si dissolved in the matrix increase the deformation resistance of the alloy. For example, when a several hundred to several thousand extrusion ratio is employed (e.g., when producing a multiport tube), the alloy exhibits significantly inferior extrudability as compared with a pure Al material. An alloy that requires a high extrusion ram pressure or has a low critical extrusion rate (i.e., the maximum extrusion rate obtained without causing breakage of the partition of the hollow area of the multiport tube) exhibits inferior extrudability. An alloy containing Mn and Si at a high concentration requires a high ram pressure as compared with a pure Al material, so that the die tends to break or wear. Moreover, productivity decreases due to a decrease in the limiting extrusion rate.
A technique has been proposed that adds Si and Mn in order to increase strength, and performs a high-temperature homogenization treatment and a low-temperature homogenization treatment in combination in order to improve extrudability to reduce the amount of solute elements dissolved in the matrix, and reduce the deformation resistance. In this case, since a large amount of solute elements are added, an improvement in extrudability (particularly an improvement in extrusion rate) is limited although an increase in strength may be achieved. Specifically, it is difficult to achieve high strength and high extrudability (i.e., productivity) at the same time.
A refrigerant leaks from a refrigerant tube (extruded tube) of an automotive heat exchanger when perforation corrosion has occurred during use. Therefore, Zn is caused to adhere to the surface of an extruded tube by thermal spraying or the like, and is diffused by brazing. A Zn diffusion layer formed in the surface area of the tube serves as a sacrificial anode for the deeper area, and suppresses corrosion in the thickness direction (i.e., increases the perforation life). In this case, the Zn application step (e.g., Zn thermal spraying) is required after extruding the tube. Moreover, a step that applies a fluoride flux required for brazing, or a step that applies a flux to the entire heat exchanger core must be performed after the Zn application step. This increases the production cost. Since a filler metal is not applied to the tube, it is necessary to use a brazing fin that is clad with a filler metal as the fin material. This also increases the production cost as compared with the case of using a bare fin material that is not clad with a filler metal.
A technique that applies a mixture of a filler metal powder and a Zn-containing flux powder to the surface of an aluminum alloy extruded refrigerant tube has been proposed in order to solve the above problems. In this case, since the filler metal, Zn, and the flux can be simultaneously applied by a single step, the production cost can be reduced. Moreover, since a bare fin material can be used as the fin material, the production cost can be further reduced. According to this technique, however, since the Zn concentration in the fin joint fillet increases due to the Zn-containing flux, preferential corrosion of the fillet occurs during use, so that the fin is separated at an early stage. The functions of the heat exchanger are impaired by separation of the fin. Moreover, since the sacrificial anode effect of the fin (that is obtained when the potential of the fin is lower than that of the tube) cannot be obtained, corrosion perforation of the tube occurs at an early stage. When the amount of the Zn-containing flux is reduced in order to prevent the above phenomenon, the amount of flux necessary for brazing becomes insufficient, so that defective brazing occurs.
As a technique that ensures brazability by maintaining the total amount of flux, a technique that applies a mixture of a filler metal powder, a Zn-containing flux powder, and a Zn-free flux powder to the surface of an aluminum alloy extruded refrigerant tube has been proposed. However, this technique mainly aims to improve brazability, and does not specify the alloy components of the extruded alloy tube that affect corrosion resistance (the alloy components are not described even in the examples). Therefore, the effect of this technique on corrosion resistance is unclear. Moreover, since the ratio of the amount of the Zn-containing flux to the amount of the Zn-free flux is too large, the Zn concentration in the fillet increases, and preferential corrosion of the fillet occurs, so that the fin is separated at an early stage.
A technique that that applies a mixture of a filler metal powder and a Zn-free flux powder to the surface of an aluminum alloy extruded refrigerant tube has been proposed in order to suppress an increase in Zn concentration in the fillet, and prevent a situation in which the fin is separated at an early stage due to preferential corrosion of the fillet. This technique causes the potential of the fin to be lower than that of the tube, and protects the tube against corrosion by utilizing the sacrificial anode effect of the fin. According to this technique, the Zn concentration in the fillet can be reduced, and a situation in which the fin is separated at an early stage due to preferential corrosion of the fillet can be prevented. However, since a sacrificial anode layer due to diffusion of Zn is not present in the tube, it is impossible to sufficiently protect the tube against corrosion in an area in which the fin is not present, or an area that is situated away from the fin (e.g., an area near the header).
In particular, when using a technique that limits the amount of Si in the tube, that causes Al—Mn—Si compounds to precipitate in the surface area of the tube due to diffusion of Si from the applied Si powder (i.e., forms an area having a low degree of Mn solid dissolution (i.e., an area having a potential lower than that of a deeper area) in the surface area of the tube), and that protects the tube against corrosion utilizing the above area as the sacrificial anode layer, the potential difference between the surface area and the deep area of the tube is small, and corrosion cannot be sufficiently prevented in a dry-wet environment.
JP-A-2005-256166, JP-A-2004-330233, JP-A-2006-255755, JP-A-2009-58139, and JP-A-2009-58167 disclose related-art technologies.